The present invention relates to lasers and more particularly to a high-gain solid-state laser or the like.
In a solid-state laser or the like, high gain is critical to achieving operation at high-average power in a compact package, such as about a 50-100 kilowatt-class or higher solid-state laser. One challenge to developing high-average power solid-state lasers is a limitation on laser gain imposed by losses caused by amplified spontaneous emission (ASE). ASE is a phenomenon wherein spontaneously emitted photons traverse the laser gain medium and are amplified before they may exit the gain medium. The favorable condition for ASE is a combination of high gain and a long path for the spontaneously emitted photons. ASE may depopulate the upper energy level in an excited laser gain medium, thereby reducing laser gain, robbing the laser of extractable power and limiting the laser's efficiency. In addition, power lost to ASE may be deposited as heat in various parts of the laser system, causing thermal stress and misalignment of other components, optical path variations and instability, reduction of beam quality and the like. ASE losses and concomitant heating can amount to several kilowatts of lost power. In pulsed lasers ASE losses may delay the laser pulse startup thereby wasting power in excited laser states. Furthermore, reflection of ASE photons at gain medium boundaries may provide feedback for parasitic oscillations that aggravate the loss of laser power. If unchecked, ASE may become large enough to deplete the upper level inversion in high-gain laser amplifiers.